In conventional magnetic disk devices, such as hard disk drives (HDDs), a head is positioned on a target track to execute writing and reading by reading servo information magnetically recorded on a disk and demodulating the information. The head comprises a write head element (writer) for recording data on the disk, and a read head element (reader) for reading data from the disk. The writer and the reader are arranged with a nominal space therebetween. Accordingly, when the data written by the writer is read by the reader, the reader is offset in a cross-track direction. The amount of offset is called a read/write (R/W) offset. R/W offsets differ from each other between individual heads. Further, when, for example, a swing actuator is employed in the magnetic disk device, the R/W offset assumes different values when the head is positioned in different radial positions on the magnetic disk. Based on the different R/W offset values, R/W offset correction is performed.
There are some methods for obtaining R/W offset values. Among these methods, there is a method of obtaining R/W offset values, based on measurement values (associated with, for example, data quality) assumed in each radial position. In this method, a R/W offset is measured for each of plural tracks.
However, to correct a track error (track deviation) due to an infralow frequency component, it is necessary to measure the R/W offset with a short pitch in the cross-track direction. Therefore, measurements for the correction of the track deviation due to the infralow frequency component require an enormous processing time.